Telecommunications equipment uses both attenuators and optical isolators. The attenuators can be used either temporarily to test power levels by adding a calibrated amount of signal loss or they can be installed permanently to properly match transmitter and receiver power levels. Fiber-optic telecommunication systems need a certain amount of optical power to work properly, but too much power can cause problems. There are cases where power may need to be restricted, for example, when couplers do not distribute signals evenly or to protect sensitive instruments. Attenuators discard surplus optical power and they can reduce signal levels in communication systems to those that the receivers can handle best. This makes it possible for all terminals in a network to use the same transmitters and receivers, even though light traveling between them suffers different losses. Adding attenuators also makes it possible to use the same terminal equipment in all parts of a local network. The terminal next to the distribution node might receive a signal 20 dB higher than one on the opposite corner of the building, but an attenuator can balance the power levels. The most common and least costly attenuators are filters that block a fixed portion of the in-coming light. These attenuators are installed in communication systems to balance power levels, and normally these attenuators will not need to be changed again.
Optical isolators are used to prevent back-reflections and other noise from reaching sensitive optical components in telecommunications systems. They act as one-way paths through which the telecommunication's frequency light passes. Optical isolators consist of, in sequence, a first or input polarizer, typically one with a vertical polarization axis to enhance the contrast ratio and clean the vertically polarized incoming light, a Faraday rotator for receiving the vertically polarized light and rotating it by 45°, and a second or output polarizer whose polarization axis is at 45° to the polarization axis of the first polarizer. The 45° rotated light from the Faraday rotator completely passes through the second polarizer to a receiver with virtually no losses, for example, an optical fiber or an analyzer. If any light is reflected backwards by receiver, the second polarizer will polarizer the back-reflected light by 45° and Faraday rotator will rotate the light from the second polarizer by an additional 45°. The back-reflected light emerging from the rotator has become horizontally polarized and will be blocked by the first polarizer that permits the passage of only vertically polarized light. Thus, any reflected light that travels in the direction opposite that of the incoming light will be extinguished. Optical isolators are important components in high-performance systems because of the way they can block noise traveling in the wrong direction through the fiber.
At the present time there is no single, integrated element, that can accomplish both these tasks; that is, act as an optical attenuator and as an optical isolator. The present disclosure provides such an element and a device that uses the element.